The active matrix device is based upon a simple principle, in which pixels are individually equipped with switch elements. If specific pixels are selected, the corresponding switch elements are turned on.
If no pixel is selected, the switch elements are left inconductive. These switch elements are formed over a glass substrate which forms part of the liquid crystal panel. Therefore, the thin film technology for forming the switch elements is important. Thin film transistors are usually formed as those elements.
In the active matrix device of the prior art, the thin film transistors are formed in the surface of an amorphous silicon thin film or a polycrystal silicon thin film, which is deposited on the glass substrate. The amorphous silicon thin film and the polycrystal silicon thin film are suited for fabricating an active matrix device of relatively large frame, because they can be easily deposited over the glass substrate by using the chemical vapor deposition (CVD) process.
Transistor elements, which are formed in the amorphous silicon thin film or the polycrystal silicon thin film, are generally of the field effect insulated gate type. At present active matrix liquid crystal devices of 3 inches to 10 inches in diagonal line using the amorphous silicon thin film are commercially produced. Since the amorphous silicon thin film can be formed at low temperature of 850.degree. C. or less, the thin film is suited for producing a liquid crystal panel having a large frame. Further, in active matrix liquid crystal devices using the polycrystal silicon thin film, small-sized liquid crystal panels about 2 inches in diagonal line are commercially produced at present.
The active matrix device of the prior art using the amorphous silicon thin film or the polycrystal silicon thin film is suited to the direct viewing type display system necessary for relatively large frame, but not necessarily suited for miniaturizing the device and heightening the density of pixels.
In recent years, however, a high demand has arisen not for the direct viewing type display device but rather for a microminiature display device or a light valve device which have miniature pixels of high density. A microminiature light valve device is used as a plane for forming a primary image of a projection type image device, for example, and can be applied for use in a projection type high-vision TV. For this application, it is possible to provide a microminiature light valve device, which has a pixel size on the order of 1 .mu.m and an overall size of several cm by using the miniature semiconductor fabricating technology.
In case, however, the existing amorphous or polycrystal thin film is used, the transistor elements of sub-micron order cannot be formed by applying the miniature semiconductor technology. In case of the amorphous silicon thin film, for example, its practical film forming temperature is about 800.degree. C. so that a high-temperature treatment necessary for the miniaturization cannot be executed. In case of the polycrystal silicon thin film, on the other hand, the crystal particles have a size of several .mu.m raising a problem in that the miniaturization of the thin film elements is necessarily restricted.
Further, since the film forming temperature of the polycrystal silicon thin film is about 600.degree. C., miniaturization technology necessary for the high-temperature treatment of at least 1000.degree. C. cannot be executed sufficiently. In the existing active matrix display device using the amorphous or the polycrystal silicon thin film, as has been described hereinbefore, there arises a problem that it is seriously difficult to realize an integration density and a chip size similar to those of the ordinary semiconductor integrated circuit elements.